1. Field of the Invention
This invention relates generally to the field of protecting composite structures from lightning strikes, and more particularly, to an appliqué having multiple conducting layers including one or more layers for protecting composite aircraft from lightning strikes and one or more layers for operational circuit functionality in aircraft systems.
2. Description of the Related Art
Since aluminum and other metals are highly conductive, a transient charge from a lightning striking a metal body discharges into the metal body with current from the discharge being distributed relatively evenly over the body. A typical lightning strike to a metal aircraft causes no or only minor damage to aircraft components. However, carbon fiber composites generally have a higher strength-to-weight ratio than aluminum and are increasingly replacing aluminum structural components.
Unfortunately, typical state of the art composites, such as Carbon Fiber Reinforced Plastic (CFRP), are approximately 2000 times more resistive than aluminum. A lightning strike to unprotected components, embedded in, passing through or attached to the CRFP aircraft skin that is generally 35-40% resin, does not dissipate as readily as in metal. Typically, temperature drives the damage level created by a lightning strike. The discharge current through composite skin resistance dissipates a burst of energy that causes the carbon fibers to become very hot, so much so, that the skin temperature becomes much hotter in a CRFP structure than in a state of the art aluminum skin. A lightning strike that may have little or no effect on an aluminum structure may damage unprotected CFRP components. For adequate lightning protection for a composite wing structure, the exterior CFRP structure must withstand an initial lightning strike of at least one hundred kiloamperes (100 kA) of discharge current without adverse affects or impact to safety-of-flight.
Furthermore, skin fasteners at an exposed surface are most vulnerable to a direct strike and such a strike is most likely to cause sparking at structural joints. Accordingly, composite structure aircraft must have some protection, especially at exposed skin fasteners. However, it is also important that this protection is economically feasible; in its initial application, in its effectiveness for minimizing resulting damage and, in subsequent consequent repair or replacement, both for continued aircraft flightworthiness and to meet economic repair targets. Unfortunately, typical approaches to minimizing the possibility ignition and reducing damage are complicated and difficult to implement in CFRP.
One approach involves selectively integrating metal (e.g., copper foil) into or onto the composite laminate at the fastening areas. However, this approach is often difficult to implement/rework with labor intensive application processes both pre and post-assembly; and, has not consistently exhibited acceptable EME protection. Copper foils, for example, have resulted in wrinkling during lay-up/cure. Drilling the laminate for fastener installation may contaminate the fuel tank with copper. Even with this additional protection, in the absence of other supporting protection (e.g., fastener collar isolation, fillet/cap sealing), the structure may still have a low sparking threshold. In addition to added complexity, integrating a conductive surface protection layer into the composite wing skin may carry with it an unacceptable weight penalty.
Numerous electrical circuits on aircraft and other vehicles require electrical conductors on or near the skin surface for operation such as deicing systems and certain antenna/sensor applications, structure health monitoring systems. Additionally, general electrical wiring for additional circuit elements, while not necessarily required to be on or near the surface require electrical conductors for operation and placement of those conductors is not critical to the operation of the system.
Application of lightning strike protection to composite structures typically requires the addition conductive layers to the surface of the structure. To enhance the usefulness of these conductive layers, incorporation of additional functionality to the layers provides economic and design efficiency benefits.
It is therefore desirable to provide lightning strike protection appliqués for aircraft which incorporate additional conductive layers for operational functionality in electrical system associated with the aircraft.